1. Field
The present invention relates generally to data communication, and more specifically to techniques for time-aligning transmissions from multiple base stations in a CDMA communication system.
2. Background
Wireless communication systems are widely deployed to provide various types of communication including voice and packet data services. These systems may be based on code division multiple access (CDMA), time division multiple access (TDMA), or some other multiple access techniques. CDMA systems may provide certain advantages over other types of system, including increased system capacity. A CDMA system is typically designed to conform to one or more standards, such as IS-95, cdma2000, and W-CDMA standards, which are known in the art and incorporated herein by reference.
A CDMA system may be operated to support voice and data communication. During a communication session (e.g., a voice call), a terminal may be in active communication with one or more base stations, which are placed in an xe2x80x9cactive setxe2x80x9d of the terminal. While in soft handover (or soft handoff), the terminal concurrently communicates with multiple base stations, which can provide diversity against deleterious path effects. The terminal may also receive signals from one or more other base stations for other types of transmission such as, for example, pilot references, pages, broadcast messages, and so on.
In accordance with the W-CDMA standard, the base stations are not required to be operated synchronously. When operated asynchronously, from a terminal""s perspective, the timing (and thus, the radio frames) of the base stations may not be aligned and the reference time of each base station may be different from those of the other base stations.
While in soft handover, a terminal concurrently receives data transmissions (i.e., radio frames) from multiple base stations. To ensure that the radio frames arrive at the terminal within a particular time window so that they can be properly processed and recovered, the W-CDMA standard provides a mechanism whereby the starting time of the terminal-specific radio frames from each base station to the terminal can be adjusted. Typically, before a new base station is added to the terminal""s active set, this base station""s timing relative to that of a reference base station is determined by the terminal and reported to the system. The system then instructs the new base station to adjust its transmit timing for the terminal such that the radio frames transmitted from this new base station are approximately aligned in time to the radio frames from the other active base stations.
For the W-CDMA standard, the time difference between a new candidate base station and a reference base station can be reported via an xe2x80x9cSFN-SFN observed time difference type 1 measurementxe2x80x9d (where SFN denotes system frame number). This measurement includes two parts. The first part provides the chip-level timing between the two base stations, which can be derived by detecting the timing of the pseudo-noise (PN) sequences used to descramble the downlink signals from these base stations. The second part provides the frame-level timing between the two base stations, which can be derived by processing (i.e., demodulating and decoding) a broadcast channel transmitted by the base stations. These two parts are encapsulated into a report message that is transmitted from the terminal to the system.
In certain W-CDMA system configurations, only the chip-level timing is required to properly time-align the radio frames of a newly added base station. This may be true, for example, if the base stations are operated synchronously and the frame-level timing is already known by the system. In this case, requiring the terminal to measure and report the frame-level timing as well as the chip-level timing (as required by the current W-CDMA standard) may degrade performance. First, if the terminal is forced to process the broadcast channel of a candidate base station before that base station can be selected for communication, then the soft handover region may be limited to only a portion of the base station""s coverage area and would be bounded by where the broadcast channel may be received. Second, the processing of the broadcast channel leads to additional delays, which may degrade performance.
There is therefore a need in the art for techniques to time-align transmissions from multiple base stations to a terminal. One such technique is to provide the required time difference (i.e., only chip-level timing or both chip-level and frame-level timing) from a terminal in a W-CDMA system for handover and other applications.
Aspects of the invention provide various schemes to time-align data transmissions from multiple base stations to a terminal. To achieve the time-alignment, time differences between the arrival times of transmissions from the base stations, as observed at the terminal, are determined and provided to the system. The system then uses the timing information to adjust the timing at the base stations such that the terminal-specific radio frames transmitted from the base stations arrive at the terminal within a particular time window.
In a first scheme, a time difference between two base stations is partitioned into a xe2x80x9cfine-resolutionxe2x80x9d part and a xe2x80x9ccoarse-resolutionxe2x80x9d part, and only the required part(s) are reported when requested. For the W-CDMA system, an SFN-SFN type 1 measurement can be partitioned into a frame-level time difference and a chip-level time difference. Whenever requested to perform and report time difference measurements for a list of one or more base stations, the terminal measures the chip-level timing for each base station in the list relative to a reference base station. Additionally, the terminal also measures the frame-level timing and includes this information in the SFN-SFN type 1 measurement only if required (e.g., as directed by the system). Otherwise, if the frame-level timing is not required, the terminal can set the frame-level part to a predetermined value, which may be a known fixed value (e.g., zero), any arbitrary value selected by the terminal and which may be ignored by the system, a value for the frame-level timing already known in advance by various means (e.g., past measurements of the same cell, transmissions from the system, and so on), or some other value.
In a second scheme, the time difference between two base stations is determined by the terminal based on partial decoding of some of the base stations received by the terminal. For the W-CDMA system, the terminal can decode a primary common control channel (P-CCPCH) for a number of base stations, which may be selected based on a particular criterion (e.g., the received signal strength). If a particular number of (e.g., two or more) decoded base stations have the same system frame number (SFN) value at a particular time instance, then a synchronous system configuration may be deduced by the terminal, and the chip-level timing but not frame-level timing is determined for the remaining base stations.
In a third scheme, the timing for the terminal is ascertained by the base stations based on an uplink transmission from the terminal. The timing information recovered by the system may then be used to adjust the timing of the downlink transmissions to the terminal.
In a fourth scheme, the time difference between two base stations is ascertained by the system based on a priori knowledge of the layout and sizes of the cells in the system. If the coverage areas of the base stations are sufficiently small, then the time uncertainty due to signal propagation delays is also small (e.g., a few chips or less). For the W-CDMA system, the time difference between a common channel frame and a dedicated channel frame may be determined (e.g., for the reference base station), and all other base stations may be associated with the same time difference between their common and dedicated channel frames.
The above schemes may be used for various applications such as hard and soft handovers, position determination, and possibly other applications. The invention further provides methods, terminal, base station, and apparatus that implement various aspects, embodiments, and features of the invention, as described in further detail below.